1. Field of the Invention
This invention relates to electrochemical cells having gas diffusion cathodes and dimensionally stable anodes.
2. Description of the Prior Art
Electrolysis of aqueous alkali metal halides to produce halogen, especially chlorine, in an electrolysis cell provided with an anode and a cathode separated by an ion permeable membrane is well known. In certain of these cells, the electrodes are bonded to and/or embedded in opposite sides of the membrane which is usually a cation exchange fluorocarbon polymer containing sulphonic acid groups or carboxylic groups as disclosed in U.S. Pat. No. 4,331,521.
The electrodes can be dimensionally stable anodes as described in U.S. Pat. No. 3,770,611, and gas diffusion cathodes containing electroconductive particles such as silver or a platinum group metal supported by carbon black or graphite and bonded with a fluorocarbon polymer which also serves to wetproof the carbon or graphite matrix.
The resulting membrane-electrode assembly is mounted in an electrolytic cell in which a substantially saturated aqueous alkali metal halide solution is fed into the anode chamber and water or dilute caustic is fed into the cathode chamber. By establishing an electrical potential between the anode and cathode, chlorine is evolved at the anode and the formation of hydrogen at the gas diffusion cathode is prevented by feeding oxygen or air to the side of the cathode opposite to that which is exposed to the electrolyte. Electrolysis of this type can be conducted at high current densities, on the order of 1,000 to 3,000 amperes per square meter of anode surface and at a voltage which is several hundred millivolts lower than with gas diffusion cathodes not bonded with a fluorocarbon.
One difficulty which is encountered in the operation of such electrolytic cells is the tendency of the gas pressure imposed on the back surface of the cathode to force the cathode against the cell membrane, thus eliminating to a large extent an area for catholyte circulation between the electrode and the cell membrane. When large electrode areas are required in the electrolytic cells, the total force required to constrain the electrode from expanding in the direction of the membrane under the applied pressure of the gas applied to the back side of the cathode is very large. If the electrode curvature must remain small, as is required in flat plate electrolytic cell designs, the stress in the supporting structures of the electrolytic cell becomes very large. Structural supporting devices are required where large electrode surfaces are used even with modest pressure differentials of 2 lbs per square inch or less.